Require document helper in Linux headless packages and verify Unix worker

docs/BUILDING.md

@@ -304,3 +304,10 @@ it with `python tools/stage_extraction_worker.py` before desktop development or
 release packaging. Release builds reject a missing helper. Headless deployments
 place the platform helper beside the CLI executable. Parser failure retains
 filename search and exposes an extraction error; there is no in-process fallback.
+
+For Linux headless archive preparation, stage the extraction helper into the
+same directory passed to `package_linux.py --runtime`:
+
+```sh
+python tools/stage_extraction_worker.py --destination .searchi/runtime/auto
+```